Motor-operated valves are being used extensively in a variety of pipes installed in nuclear power plants and other facilities for many years.
A motor-operated valve in such applications comprises: a valve disc for opening and closing a passage in a pipe; a worm rotatably driven by motor power such as an electric motor; a link mechanism (a gear, a drive sleeve, a stem nut, a valve stem, etc.) which opens and closes the valve disc with the rotational driving force transmitted from the worm; and a spring cartridge containing disc springs which expand or compress in response to reaction force acting on the worm in its axial direction from the link mechanism.
The spring cartridge contains disc springs and is provided to prevent the worm from delivering excessive rotational driving force (torque) to the link mechanism. The spring cartridge is also referred to as a “spring pack,” “torque spring cartridge,” or “torque spring pack.”
The spring cartridge comprises a shaft, a plurality of disc springs, and two compression plates. The shaft moves axially together with the worm. The disc springs are fitted around the shaft with the shaft penetrating the center of the disc springs. The compression plates hold the disc springs in the axial direction from both ends. Each of the compression plates is able to slide together with the shaft only in the direction into which the disc springs are compressed. The compression plates hold the disc springs compressed with a predetermined load applied in advance.
This load holding the disc springs pre-compressed is referred to as the “preload” of the spring cartridge.
When a motor or the like is operated to move the valve disc from the open position to the close position, the driving force from the motor rotates the worm and drives the link mechanism, the link mechanism comprising a gear engaging the worm, a drive sleeve, a stem nut, a valve stem, etc. Consequently the valve disc descends and is closed.
Reverse rotation of the worm raises and opens the valve disc.
With the rotation of the worm, the driving force is transmitted to the link mechanism, which in turn drives the valve disc. The valve disc, when closed, contacts a valve seat located in the valve body. When this contact occurs, the valve disc begins to receive reaction force from where it has contacted.
This reaction force is then transmitted to the worm by way of the link mechanism and acts on the worm as a load to displace the worm in its axial direction.
On the other hand, the spring cartridge shaft and one of the compression plates move integrally with the worm. This movement compresses the disc springs fitted around the spring cartridge shaft by the amount which corresponds to the displacement of the compression plate.
However, the spring cartridge urges the worm in its axial direction with the preload. Therefore, the worm will not begin to move until a component of the aforementioned reaction force acting on the worm exceeds the preload. When the reaction force acting on the worm exceeds the preload, the worm is displaced in a direction which compresses the disc springs in the spring cartridge according to the compression force on the spring cartridge.
That is to say, the worm begins to move against the urging force when the preload is exceeded by the reaction force. The reaction force continuously increases until the valve disc reaches a predetermined close position after it contacts the valve seat.
If the valve disc continues to be closed after it reaches the predetermined close position, damage may be caused to the valve disc, the interface between the worm and the link mechanism, or other parts of the valve disc. To prevent this from happening, the rotation of the worm need to be stopped.
Accordingly, a limit switch (a torque switch) is provided at a position to which the worm will move when the valve disc reaches a predetermined close position, so that the motor operation can stop.
This means that the set point for actuating the torque switch provided on the worm is determined by the amount of worm displacement (in the compression direction).
When the Hooke's Law is applied to the spring cartridge, an equation, F=k·x, exists. F is a load, k is a spring constant, and x is an amount of compression.
The preload F0 of the spring cartridge is then given by F0=k·x0, where x0 is the amount of pre-compression. This preload F0 acts on the worm as an urging force.
Accordingly, an urging force, Fs, acting on the worm when the torque switch is actuated is given by:
                    Fs        =                              F            ⁢                                                  ⁢            0                    +                      k            ·            xs                                                            =                      k            ·                          (                                                x                  ⁢                                                                          ⁢                  0                                +                xs                            )                                      ,            where xs is the amount of worm displacement set to actuate the torque switch.
Thus, the actuating point of the torque switch is substantially defined by the load Fs acting on the worm.
However, the aforementioned disc springs in the spring cartridge may experience wear or similar degradation due to ageing or some other causes.
When this happens, the amount of pre-compression, x0, on the disc springs decreases by Δx corresponding to a reduction in the length caused by wear. In this case, the pre-compression amount x1is given by:x1=x0−Δx. The preload F1 is given by;
                              F          ⁢                                          ⁢          1                =                              k            ·            x                    ⁢                                          ⁢          1                                        =                  k          ·                      (                                          x                ⁢                                                                  ⁢                0                            -                              Δ                ⁢                                                                  ⁢                x                                      )                                                  =                              F            ⁢                                                  ⁢            0                    -                                    k              ·              Δ                        ⁢                                                  ⁢                          x              .                                          As shown, the preload after wear occurred becomes lower than the preload F0 before the wear occurred.
Load Fs′ at the actuating point of the torque switch (x=xs) is then expressed by:
                              Fs          ′                =                              F            ⁢                                                  ⁢            1                    +                      k            ·            xs                                                  =                              (                                          F                ⁢                                                                  ⁢                0                            -                                                k                  ·                  Δ                                ⁢                                                                  ⁢                x                                      )                    +                      k            ·            xs                                                  =                  Fs          -                                    k              ·              Δ                        ⁢                                                  ⁢                          x              .                                          As shown, the load Fs′ after wear occurred becomes lower than the load Fs before the wear occurred by ΔF (=k·Δx).
This can cause the torque switch to be actuated before the valve disc reaches a predetermined close position, or with an excessive margin for a specified allowable strength. This in turn may prevent a motor-operated valve from operating properly in accordance with its characteristics.
The description provided above assumes a valve operation in which the valve disc moves from an open position to a close position. Similar actions and effects apply to a valve operation in which the valve disc moves from a close position to an open position, with a resulting decrease in the load acting on the torque switch.
Consequently, a variety of motor-operated valve diagnostic apparatuses have heretofore been proposed. The proposed apparatuses often determine the torque curve (the section at which the compression amount is zero represents the preload) representing a relation between the load (or torque) acing on the spring cartridge and the compression amount of the spring cartridge (excluding the pre-compression amount), and then, based on the torque curve thus determined, produce a diagnosis on whether or not a motor-operated valve is operating properly.
For measuring torque, one apparatus uses a strain sensor, for example. In such an apparatus, the strain sensor is permanently provided on a motor-operated valve as an integral part of the spring cartridge (built-in torque sensor; an example can be found in International Publication Gazette WO95/14186). The strain sensor is for directly measuring stress (or torque) acting on the compression plate of the spring cartridge from the worm. Another diagnostic apparatus measures torque using a load cell together with a displacement sensor (externally-attached torque sensor; examples can be found in Japanese patent No. 2982090, FIG. 1 and other literature). In this case, a calibrated load cell is mounted on a motor-operated valve from outside its casing, with the use of an adapter placed between the load cell and the motor-operated valve, in a way that the load cell hits the compression plate of the spring cartridge. The displacement sensor is for detecting displacement of the spring cartridge shaft.
The externally-attached torque sensor as described above uses the displacement sensor to detect displacement of the spring cartridge shaft and the load cell to measure stress applied from the compression plate, when the spring cartridge shaft moves toward the load cell to compress the disc springs. From the result of this, the preload and the spring constant are determined. When the shaft moves away from the load cell together with the compression plate located at the shaft end and compresses the disc springs, the displacement of the shaft is detected by the displacement sensor, while the stress is detected using the detected displacement and the spring constant already obtained.
In another diagnostic apparatus, a spring cartridge is taken out of the casing of a motor-operated valve. Measurement is then made on the spring cartridge alone to determine the elastic characteristics thereof. In yet another diagnostic apparatus, a diagnosis of a motor-operated valve is produced based only on the detection of worm displacement.